The increasing demand for high-speed voice and data communications has led to an increased reliance on optical communications, especially optical fiber communications. The use of optical signals as a vehicle to carry channeled information at high speed is preferred in many instances to carrying channeled information at other electromagnetic wavelengths/frequencies in media such as microwave transmission lines, coaxial cable lines, and twisted copper pair transmission lines. Advantages of optical media include higher channel capacities (bandwidth), greater immunity to electromagnetic interference, and lower propagation loss. In fact, it is common for high-speed optical systems to have signal rates in the range of approximately several megabits per second (Mbit/s) to approximately several tens of gigabits per second (Gbit/s), and greater. However, as the communication capacity is further increased to transmit greater amounts of information at greater rates over fiber, maintaining signal integrity can be exceedingly challenging.
The emergence of optical communications as a useful vehicle for short and long-haul data and voice communications has led to the development of a variety of optical amplifiers. One type of optical amplifier is the rare-earth element optical amplifier (rare-earth doped amplifier). One such rare-earth doped amplifier is based on erbium-doped silica fiber. The erbium doped fiber amplifier (EDFA) has gained great acceptance in the telecommunications industry. The erbium-doped fiber amplifier has a number of characteristics which make it an excellent amplifier for optical communications. These characteristics include polarization-independent gain, low interchannel cross-talk, wide optical bandwidth, and low-noise generation. In brief, the EDFA offers a useful way to compensate for signal propagation loses along high-speed fiber-optic links.
Erbium-doped fiber amplifiers (EDFA) are useful in a variety of optical transmission systems. One way to more efficiently use available resources in the quest for high-speed information transmission is known as multiplexing. One particular type of multiplexing is wavelength division multiplexing (WDM). In WDM, several information streams (voice and/or data streams) share a particular transmission medium, such as an optical fiber. Each high-speed information channel is transmitted at a designated wavelength along the optical fiber. At the receiver end, the interleaved channels are separated (de-multiplexed) and may be further processed by electronics. (By convention, when the number of channels transmitted by such a multiplexing technique exceeds approximately four, the technique is referred to dense WDM or DWDM). As WDM gains popularity, optical amplifiers may be required to give requisite signal boost to preserve signal quality, particularly in long-haul applications.
Typically, optical amplifiers used in WDM based systems must satisfy certain requirements. One of the requirements is that the gain of the amplifier over the operating spectrum is substantially flat with low gain tilt and a low noise figure. This requirement is often referred to as gain flatness. As can be appreciated, gain flatness is required to avoid the dominance of the power of one or more channels over the others.
Another requirement of the optical amplifier is good transient characteristics. This requirement is related to the sensitivity of the surviving signals present in the optical network to the adding or dropping of some other signals (channels). When additional channels are added, the total optical power may experience a large upward transient spike that may last up to a millisecond causing a temporary increase in the bit-error-rate (BER). If the channels are dropped, total optical power may experience a large downward transient spike. This may also increase (BER) due to effects such as receiver overload or some nonlinear phenomena, such as stimulated Brillouin scattering. In addition to the above described affects, the amplifier may exhibit a permanent shift in gain or an unwanted power offset.
To fulfill the above illustrative requirements, it is necessary to control the optical amplifier during operation. While control mechanisms and schemes have been incorporated in conventional optical amplifiers, they have shortcomings in deployed systems. To this end, conventional controllers lack the capability to control fast gain and output power transients. Control of these transients is useful in order to avoid cross-talk between the channels caused by the adding or dropping of channels, or by changing set-point values for the gain or output power in variable gain amplifiers.
Accordingly, what is need is a controller and its method of use which overcomes the drawbacks of conventional controllers described above.